Various processes have been proposed for expanding tobacco wherein the tobacco is impregnated with or otherwise treated with various agents and then subjected to an expansion treatment. For example, tobacco has been contacted with a gas under somewhat greater than atmospheric pressure, followed by a release of the pressure, whereby the tobacco cells are expanded to increase the volume of the treated tobacco. Other methods which have been employed or suggested have included the treatment of tobacco with various liquids, such as water or relatively volatile organic liquids, to impregnate the tobacco with the same, after which the liquids are driven off to expand the tobacco. Additional methods which have been suggested have included the treatment of tobacco with solid materials which, when heated, decompose to produce gases which serve to expand the tobacco. Other methods include the treatment of tobacco with gas-containing liquids, such as carbon dioxide-containing water, under pressure to incorporate the gas in the tobacco and then the tobacco impregnated therewith is heated or the pressure thereon is reduced to thereby expand the tobacco. Additional techniques have been developed for expanding tobacco which involve the treatment of tobacco with gases which react to form solid chemical reaction products within the tobacco, which solid reaction products may then decompose by heat to produce gases within the tobacco which cause expansion of the tobacco upon their release. More specifically:
A patent to Wilford J. Hawkins, U.S. Pat. No. 1,789,435, granted in 1931, describes a method and apparatus for expanding the volume of tobacco in order to make up the loss of weight caused in curing tobacco leaf. To accomplish this object, the cured and conditioned tobacco is contacted with a gas, which may be air, carbon dioxide or steam under about 20 pounds of pressure and the pressure is then relieved, whereby the tobacco tends to expand. The patent states that the volume of the tobacco may, by that process, be increased to the extent of about 5-15%.
An alien property custodian document No. 304,214 to Joachim Bohme, dated 1943, indicates that tobacco can be expanded using a high-frequency generator but that there are limitations to the degree of expansion which can be achieved without affecting the quality of the tobacco.
A patent to Frank J. Sowa, U.S. Pat. No. 2,596,183, granted in 1952, sets forth a method for increasing the volume of shredded tobacco by adding additional water to the tobacco to cause the tobacco to swell and thereafter heating the moisture-containing tobacco, whereby the moisture evaporates and the resulting moisture vapor causes expansion of the tobacco.
A series of patents to Roger Z. de la Burde, U.S. Pat. Nos. 3,409,022, 3,409,023, 3,409,027 and 3,409,028, granted in 1968, relate to various processes for enhancing the utility of tobacco stems for use in smoking products by subjecting the stems to expansion operations utilizing various types of heat treatment or microwave energy.
A patent to John D. Hind, U.S. Pat. No. 3,425,425, granted in 1969, which is assigned to the same assignee as the assignee of the present invention, relates to the use of carbohydrates to improve the puffing of tobacco stems. In that process, tobacco stems are soaked in an aqueous solution of carbohydrates and then heated to puff the stems. The carbohydrate solution may also contain organic acids and/or certain salts which are used to improve the flavor and smoking qualities of the stems.
A publication in the "Tobacco Reporter" of November 1969 by P. S. Meyer describes and summarizes tobacco puffing or expansion procedures or investigations for expanding and manipulating tobacco for purposes of reducing costs and also as the means for reducing the "tar" content by reduction in the delivery of smoke. Mention is made in this publication of puffing tobacco by different procedures including the use of halogenated hydrocarbons, low pressure or vacuum operation, or high pressure steam treatment that causes leaf expansion from inside the cell when outside pressure is suddenly released. Mention is also made in this publication of freeze-drying tobacco which can also be employed to obtain an increase in volume.
Since the above-mentioned "Tobacco Reporter" article was published, a number of tobacco expansion techniques, including some of the techniques described in the article, have been described in patents and/or published patent applications. For example, U.S. Pat. No. 3,524,452 to Glenn P. Moser et al. and U.S. Pat. No. 3,524,451 to James D. Frederickson, both issued in 1970, relate to the expansion of tobacco using a volatile organic liquid, such as a halogenated hydrocarbon.
U.S. Pat. No. 3,734,104 to William M. Buchanan et al., which is assigned to the same assignee as the assignee of the present invention, issued in 1973, relates to a particular process for the expansion of tobacco stems.
U.S. Pat. No. 3,710,802 to William H. Johnson, issued in 1973 and British Specification 1,293,735 to American Brands, Inc., published in 1972, both relate to freeze-drying methods for expanding tobacco.
South African applications 70/8291 and 70/8292 to R. J. Reynolds Tobacco Company, both filed in 1970, relate to tobacco expansion employing chemical compounds which decompose to form a gas or with inert solutions of a gas under pressure to maintain the gas in solution until it impregnates the tobacco.
A patent to Robert G. Armstrong et al., U.S. Pat. No. 3,771,533, issued in 1973, which is assigned to the same assignee as the assignee of the present invention, involves a treatment of tobacco with carbon dioxide and ammonia gases, whereby the tobacco is saturated with these gases and ammonium carbonate is formed in situ. The ammonium carbonate is thereafter decomposed by heat to release the gases within the tobacco cells and to cause expansion of the tobacco.
Despite all of the above-described advances in the art, no completely satisfactory process has been found. The difficulty with the various earlier suggestions for expanding tobacco is that, in many cases, the volume is only slightly or at best only moderately increased. For example, freeze-drying operations have the disadvantages of requiring elaborate and expensive equipment and very substantial operating costs. With respect to the teaching of using heat energy, infrared or radiant microwave energy to expand tobacco stems, the difficulty is that while stems respond to these heating procedures, tobacco leaf has not generally been found to respond effectively to this type of process.
The use of special expanding agents, for example, halogenated hydrocarbons, such as are mentioned in the Meyer publication for expanding tobacco, is also not completely satisfactory, because some of the materials employed are not always desired as additives. Furthermore, the introduction, in considerable concentration, of materials which are foreign to tobacco presents the problem of removing the expansion agent after the treatment has been completed in order to avoid affecting aroma and other properties of the smoke due to extraneous substances used or developed from the combustion of the treated tobacco.
The use of carbonated water has also not been found to be effective.
While the method of employing ammonia and carbon dioxide gases is an improvement over the earlier described methods, it is not completely satisfactory under some circumstances, in that undesired deposition of salts can result during the process.
Carbon dioxide has been used in the food industry as a coolant and, more recently, has been suggested as an extractant for food flavors. It has also been described in German Offenlegungsschrift 2,142,205 (Anmeldetag; 23 August 1971) for use, in either gaseous or liquid form, to extract aromatic materials from tobacco. However, there has been no suggestion, in connection with these uses, of the use of gaseous carbon dioxide for the expansion of these materials.
A process employing liquid carbon dioxide has been found to overcome many of the disadvantages of the abovementioned prior art processes. The expansion of tobacco, using liquid carbon dioxide is described in Belgium Patent 821,568, which corresponds to U.S. application Ser. No. 441,767 to de la Burde and Aument and is assigned to the same assignee as the present application and in Belgium Patent 825,133 to Airco, Inc. This process may be described as a process for expanding tobacco comprising the steps of (1) contacting the tobacco with liquid carbon dioxide (CO.sub.2) to impregnate the tobacco with the liquid carbon dioxide, (2) subjecting the liquid carbon dioxide-impregnated tobacco to conditions such that the liquid carbon dioxide is converted to solid carbon dioxide and (3) thereafter subjecting the solid carbon dioxide-containing tobacco to conditions whereby the solid carbon dioxide is vaporized to cause expansion of the tobacco. Improvements in this last process are described in U.S. application Ser. No. 822,793 to Sykes et al., assigned to the same assignee as the instant application. According to various embodiments of the invention described in this last application, the moisture content of the tobacco employed in the first step of the basic process is controlled and/or the excess liquid carbon dioxide is drained off after the first step and before step 2 and/or the output moisture of the product recovered from the third step of the basic process is controlled.
In earlier work with gaseous CO.sub.2, at pressures of about 100 psia, it was found that only minute amounts of carbon dioxide gas could be incorporated in the tobacco and held there sufficiently long for the tobacco to be heated and expanded. Thus, no substantial improvement over the prior art was found and gaseous CO.sub.2 was, therefore, believed to be much less effective as an expanding agent than the liquid carbon dioxide employed in the expansion process of the above-mentioned U.S. application Ser. No. 441,767. In copending U.S. application Ser. No. 891,468 to de la Burde et al., assigned to the same assignee as the instant application, a process is described wherein gaseous carbon dioxide remains in the tobacco in an amount of one percent or more to form a product which can thus be expanded. That process may be described as follows: A process for expanding tobacco to achieve at least about 50 percent increase in cylinder volume, comprising the steps of (1) impregnating tobacco with gaseous carbon dioxide under a pressure of at least about 250 psig and at sufficient temperature that substantially all of the carbon dioxide is maintained in gaseous form, (2) decreasing the pressure on the carbon dioxide-impregnated tobacco and (3) heating the impregnated tobacco under conditions effective to liberate the carbon dioxide therein so as to cause expansion of the tobacco. Copending U.S. application Ser. No. 891,290 to Utsch and assigned to the same assignee as the instant application, describes an improvement in this last process whereby the gaseous carbon dioxide remains in the tobacco in an amount of as high as three percent or more to form a product which can then be expanded. This improvement comprises cooling the gaseous carbon dioxide and tobacco system in step (1) above to a temperature close to the saturation temperature of carbon dioxide but not lower than -23.degree. C. The conditions during this cooling are such that the carbon dioxide is not condensed to any significant degree, but rather remains substantially in the gaseous state.
We have now found that satisfactory impregnation of tobacco with carbon dioxide can be achieved employing relatively low pressures followed by supercooling. Unexpectedly superior results and advantages can be achieved upon expansion of tobacco impregnated with carbon dioxide in the manner set forth in the present specification.